In recent years, a sealed secondary battery (which may be hereinafter simply referred to as “secondary battery”) represented by a lithium ion secondary battery is used as a power source not only for a mobile apparatus such as a portable phone or a notebook personal computer but also for an electrically driven vehicle such as an electric automobile or a hybrid car. A cell constituting the secondary battery includes an electrode group formed by winding or stacking a positive electrode and a negative electrode with a separator interposed therebetween, and an outer casing that accommodates the electrode group. Generally, a laminate film or a metal can is used as the outer casing, and the electrode group is accommodated together with an electrolytic solution in a sealed space located in the inside of the outer casing.
For use that requires a high voltage such as in a power source for the above-described electrically driven vehicles, the secondary battery is used in a form of a battery module or a battery pack that includes a plurality of cells. In the battery module, a plurality of cells connected in series is accommodated in a case. For example, four cells are connected in an array of two in parallel by two in series or in an array of four in series. Also, in the battery pack, various apparatus such as a controller are accommodated in the case in addition to the plurality of battery modules connected in series. In the secondary battery used in the power source for an electrically driven vehicle, the case of the battery pack is formed in a shape that is suitable for mounting on the vehicle.
In the meantime, such a secondary battery has a problem that, when the electrolytic solution is decomposed due to overcharging or the like, the cells swell in accordance with the rise in internal pressure caused by the decomposition gas, whereby the secondary battery is deformed. In that case, ignition occurs unless the charging current or the discharging current is stopped. In the worst case, the ignition results in rupture of the secondary battery. Therefore, in preventing rupture of the secondary battery, it is important to detect deformation of the secondary battery resulting from swelling of the cells with a high degree of sensitivity so that the charging current or the discharging current can be stopped at a suitable time.
Patent Document 1 discloses a method in which, in a battery module having a plurality of cells, a sensor insertion space is formed in the battery module for mounting a temperature sensor that senses a temperature of the cells. However, by such a technique of using the temperature sensor, a space for mounting the sensor is separately provided, so that the capacity of the secondary battery is restricted to an extent more than needed.
Also, Patent Document 2 discloses a method of reducing the charging current or the discharging current of the secondary battery by bonding a strain gauge onto a surface of a case (one example of the outer casing) of the cell and detecting change in a resistance value of the strain gauge in accordance with the swelling of the case. However, by such a technique of using the strain gauge, there is a fear that the stability of the sensor may lower due to fluctuation of the sensor characteristics resulting from positional shift of the strain gauge or the like by vibration particularly during the use for a long period of time.